Conventionally, as a lightweight and high-speed memory system capable of power-saving, there is a flash memory drive (SSD: solid state drive) equipped with integrated NAND flash memories. As an operation mode of such SSD, a SLC (single level cell) type or MLC (multiple level cell) type access reliability model has been used. However, a reliability model for switching operation between a SLC access mode (hereinafter to be referred to as a SLC mode) and a MLC access mode (hereinafter to be referred to as a MLC mode) is not still established. In order to provide a uniform reliability model and install the model on a SSD system, a circuit design capable of predicting lifetime of NAND chips in a SSD is necessary.